Many systems, among which civil and military navaid systems (TACAN, VOR, . . . ), radar systems and so on, require radio signal transmission with a moving radiation pattern (generally in a periodic fashion), very often the transmission rotating around about an axis.
Up to some decades ago, such motion effect was obtained by physically moving the transmission antenna. This technique posed a problem for the reliability and accuracy of such systems.
As soon as the electronic technology made it possible, such systems were slowly replaced with more modern systems in which the motion of the radiation pattern was simulated through an antenna array fed with electrical signals having phase and/or amplitude variable with time in a suitable relation with each other. In such systems it is therefore necessary to provide antennas with feeding devices capable of generating electrical signals having the suitable characteristics. This often involves the need for switching the same electrical signal from one antenna to another as it occurs e.g. in Doppler VOR systems. It is well known that, in order to obtain a perfect simulation of the motion, such switching must be gradual.
In the course of the description reference will only be made to Doppler VOR systems for simplicity but, as already stated, the same problems can be encountered in other systems.
A VOR (Very high frequency OmniRange) system allows an aircraft equipped with a suitable receiver to be provided with bearing information relative to a ground beacon whose geographical position is known.
The VOR radiates in a frequency band (108-118 MHz) a signal whose quality largely depends upon the place in which the VOR is installed. In fact, the omnidirectional radiation is subject to distortion due to undesired reflections by objects.
For this reason, a conventional VOR is advantageously replaced with a Doppler VOR.
In a Doppler VOR system, a "reference" signal corresponding to a VHF carrier amplitude modulated by a sine wave having a frequency of 30 Hz is transmitted according to an omnidirectional pattern, and a "variable" signal corresponding to two sidebands at +/-9960 Hz of the VHF carrier frequency is modulated by a sine wave having a frequency of 30 Hz, according to a directional pattern carrying azimuth information. These two sidebands are emitted independently from each other through separated circuits. The omnidirectional pattern is radiated through a central antenna while the other pattern is obtained by rotating two antennas on a circle, positioned diametrally opposite from each other and each fed by one of the two sidebands at +/-9960 Hz of the carrier. The revolution frequency of these antennas is 30 Hz.
For practical reasons, as already mentioned, the physical rotation of the antennas which emit the two sidebands is simulated through the gradual switching of the D-VOR signal between the fixed antennas of a circular array in such a way as to simulate the progressive motion of the emitting point from one antenna to the next one. More specifically, one carries out, in general, the switching of a radiofrequency signal amplitude modulated in accordance with a suitable time function, called a blending function, which is able to attenuate the spurious modulations that are generated because of the discretization of the antennas themselves.
Several solutions for the emission of the two sidebands of the amplitude modulation of the VHF carrier exist at present; in particular the emission of only one sideband, of two sidebands alternately, and of two sidebands simultaneously over two antennas or two diametrically opposed antenna groups. The present invention can be applied to each of these solutions.
More details on operation of D-VOR systems and problems connected with their design, in particular of the blending function, of the distribution device and of the antenna array, are described by B. R. Johnson and J. G. N. Lee in "A Double-Sideband Doppler VHF Omnirange Beacon" - AWA Technical Review, Vol. 15, No. 1, 1973 and by F. L. van der Berg in "The Philips Doppler VOR Beacon RN200" - Philips Telecommunication review, vol. 34, No. 1, April 1976.
It is clear that the design of a new feeding device for a Doppler VOR system will have to face well known problems, such as the choice of a suitable blending function, the generation of the same, and the complexity of the distribution device.
The ideal blending function is notoriously sinX/X (provided that there is no interaction between the antennas of the array) but, given the physical impossibility of realizing this function, approximations thereof are used. In the above-mentioned articles, the use of the function sinX is limited to a half period.
From U.S. Pat. No. 3,896,444, a blending function in a Doppler VOR system is known that is an approximation of the function sinX/X, restricted to the three center lobes, obtained through combination of two amplitude modulated signals according to a rectified cosX function.